The optical length must be short in the above mentioned imaging lens. In other words, to construct an imaging lens, the ratio of the optical length to a focal length of the imaging lens must be minimized. The optical length here refers to a length defined as a distance from the entrance surface of the imaging lens at the object side to the image formation surface (light receiving surface of the solid-state image sensor). Hereafter an imaging lens of which ratio of the optical length to the focal length is small may be referred to as a “compact imaging lens”, and implementing a compact imaging lens may be referred to as “compacting the imaging lens”. In the case of a portable telephone, for example, at least this optical length must be smaller than the thickness of the portable telephone main unit.
Back focus, which is defined as a distance from the outgoing surface to the image sensing surface, on the other hand, is preferably at the maximum. In other words, when an imaging lens is designed, the ratio of the back focus to the focal length must be maximized. This is because of the need to insert a filter, cover glass, and other components between the pickup lens and the pickup surface.
In addition to this, it is naturally demanded for an imaging lens that various aberrations are corrected to be small enough that the distortion of an image is not visually recognized, and the integration density of the elements in minimum units for detecting light (also called “pixels”), which are arranged in a matrix on the light receiving surface of the CCD image sensor, for example, is sufficiently satisfied. In other words, various aberrations must be well corrected. Hereafter an image when various aberrations are well corrected may be referred to as a “good image”.
As listed below, three lens-configuration imaging lenses, which use such a solid-state image sensor as a CCD image sensor or a CMOS image sensor, and are suitable for an imaging device of a portable telephone, have been disclosed.
Of these imaging lenses, the following first to sixth imaging lenses have been disclosed as a three lens-configuration lens belonging to a first category (see Patent Documents 1 to 6).
The first imaging lens is comprised of an aperture stop, a first lens, a second lens and a third lens arranged in this sequence from the object side. The first lens is a meniscus lens having a positive refractive power, of which convex surface is facing the image surface side near the optical axis. The second lens is a meniscus lens having a negative refractive power, of which convex surface is facing the object side near the optical axis, and at least the first surface of the second lens is formed in an aspherical shape. The third lens is a lens having a positive refractive power, of which convex surface is formed at the image surface side near the optical axis. In the first imaging lens, the second surface at the image surface side of the first lens is a convex surface, and the first surface of the first lens is a concave surface or plane surface, and because of this, the first imaging lens has a wide angle of view. The first imaging lens can correct the aberration well, and can be manufactured easily (see Patent Document 1).
The second imaging lens is a single focus lens which has a simple configuration of an aperture stop and three lenses, and is comprised of a first lens having a positive refractive power of which at least one surface is aspherical, a second lens having a negative refractive power, and a third lens having a positive refractive power of which at least one surface is aspherical, arranged in this sequence from the object side. The second imaging lens has a configuration where the refractive power is distributed into positive, negative and positive sequentially from the object side, and the aperture stop is between the first lens L1 and the second lens L2, which is a configuration appropriate for acquiring good optical performance as a three lens-configuration imaging lens. Also by this configuration, a distance from the exit pupil to the image formation position can be long. This means that the angle formed by a principal ray of each beam, emitted from the final surface of the lens system, and the optical axis, can be small, that is, the telecentric performance can be better, color irregularity can be prevented, and good optical performance can be acquired (see Patent Document 2).
The third imaging lens is comprised of a first lens having a positive refractive power, of which a concave surface is formed at the object side near the optical axis, an aperture stop, a second lens having a negative refractive power, and a third lens having a positive refractive power, arranged in this sequence from the object side. The ratio of a focal length f1 of the first lens to a focal length f3 of the third lens (f1/f3) is set to be 1.2 or less and 0.8 or more. Because of this configuration, a wide angle of view can be secured while maintaining a desired optical performance even if the image sensor becomes small. This imaging lens can correct each aberration well, and make the focal length shorter. And this imaging lens can also downsize the entire optical system, and can easily be manufactured (see Patent Document 3).
The fourth lens is comprised of a first lens, a second lens and a third lens, arranged in this sequence from the object side. The first lens is a meniscus lens formed of plastic with at least one aspherical surface, having a weak refractive power, of which convex surface is facing the object side. The second lens is a meniscus lens formed of plastic with at least one aspherical surface, having a weak refractive power, of which concave surface is facing the object side. The third lens is a lens formed of glass having a positive refractive power. The first lens and the second lens are plastic lenses, but the refractive powers thereof are weakened so that most of the refractive power is distributed to the third lens, therefore even if this fourth imaging lens is a compact three lens configuration, the shift of focus due to a change in temperature can be suppressed.
The convex surface of the first lens faces the object side in the fourth imaging lens. This is for preventing an increase of the negative distortion aberration. The concave surface of the first lens and the concave surface of the second lens face each other so as to decrease the generation of spherical distortion and comatic aberration. The third lens plays a role of achieving an optimum focal length by converging the beams, and correcting the image surface, which became “over” by the first and second lens, to be “under”. Because of this configuration, an image formation lens with good image forming performance can be downsized at lower cost, and can easily suppress a focus shift due to a temperature change, can be implemented using plastic lenses (see Patent Document 4).
The fifth imaging lens is comprised of a first lens, an aperture stop, a second lens and a third lens, arranged in this sequence from the object side. The first lens is a lens having a positive refractive power, of which the object side surface is a convex surface. The second lens is a meniscus lens formed of plastic material of which at least one surface is aspherical, and which has a positive or negative refractive power, and of which concave surface is facing the object side near the optical axis. The third lens is a meniscus lens having a positive refractive power of which both surfaces are aspherical, and convex surface faces the object side. Because of this configuration, high optical performance can be acquired while decreasing cost, and application is possible not only to module cameras for portable telephones, but also to digital cameras (see Patent Document 5).
The sixth imaging lens is comprised of a first lens which is biconvex, a second lens, which is a meniscus lens of which concave surface is facing the object side, and a third lens, which is a meniscus lens of which convex surface is facing the object side. Since the second lens and the third lens are meniscus lenses, and the concave surface of the second lens faces the object side and the convex surface of the third lens faces the object side, each aberration can be corrected well, the overall length of the lens system can be decreased, and high resolution can be implemented (see Patent Document 6). The embodiment disclosed in Patent Document 6 shows that the third lens has a positive refractive power. In other words, the third lens is a meniscus lens of which convex surface is facing the object side, where the radius of curvature at the object side is set to be shorter than the radius of curvature at the image side.
As mentioned above, the imaging lenses belonging to the first category represented by the first to sixth imaging lenses commonly comprise the third lens having a positive refractive power. Therefore in the case of the imaging lenses belonging to the first category, increasing the ratio of the back focus to optical length is difficult. In other words, in the case of imaging lenses belonging to the first category, if sufficient length is secured for back focus, the optical length becomes long, and compacting imaging lenses is difficult.
As a three lens-configuration imaging lens belonging to the second category, the following seventh imaging lens has been disclosed (see Patent Document 7).
The seventh imaging lens is a front shutter type single focus lens comprised of an aperture stop, a first lens, a second lens and a third lens arranged in this sequence from the object side. The first lens is a lens of which at least one surface is aspherical, and object side is a concave surface, and which has a negative refractive power. The second lens is a lens having a positive refractive power. The third lens is a lens of which at least one surface is aspherical, and object side is a concave surface, and which has a negative refractive power. Because of this configuration, a front shutter type single focus lens, which can be made compact while correcting aberrations well, can be implemented using a three lens-configuration in an imaging lens for a compact size image sensor for a digital camera, for example (see Patent Document 7).
However the seventh imaging lens is a lens of which the first lens disposed at the object side has a negative refractive power, so sufficient compacting is difficult.
As three lens-configuration imaging lenses belonging to a third category, the following eighth to tenth imaging lenses have been disclosed (see Patent Documents 8 to 10).
The eighth imaging lens is comprised of a first lens, which is a meniscus lens having a positive refractive power, of which convex surface is facing the object side, a second lens, which is a meniscus lens having a negative refractive power, and a third lens having a positive refractive power, arranged in this sequence from the object side. The Abbe number of the second lens is set to less than 50. Because of this configuration, shading can be prevented by setting the maximum exit angle to the element surface of the light receiving element to be smaller than the angle of view, and aberrations can be corrected so as to support a solid-state image sensor which is comprised of high density pixels at the mega bit order, and a imaging lens which is further compact and light weight can be implemented (see Patent Document 8).
The ninth imaging lens is comprised of a first lens having a positive refractive power, of which convex surface is facing the object side, a second lens, which is a meniscus lens having a negative refractive power, of which convex surface is facing the image side, and a third lens having a positive refractive power, of which convex surface is facing the object side, arranged in this sequence from the object side to the image side. The Abbe number of the second lens is set to a 20 to 40 range. Because of this configuration, an imaging lens where compactness and light weight are implemented while correcting such aberrations as chromatic aberration, distortion aberration and curvature of image surface well, back focus is appropriately secured, and telecentric performance can be maintained by decreasing the incident angle of principal rays to the sensor surface of the image sensor, can be implemented (see Patent Document 9).
The tenth imaging lens is comprised of a first lens having mainly a positive refractive power, of which convex surface is facing the object side, a second lens, which is a meniscus lens of which concave surface is facing the object side, and a third lens which functions as a corrective lens, arranged in this sequence from the object side to the image surface side. Because of this configuration, the overall length of the lenses can be effectively decreases while correcting various aberrations well, and an imaging lens where compactness and light weight are possible, while maintaining a high optical performance, is implemented.
By setting the Abbe number of the first lens to a value in a 40 to 72 range, the Abbe number of the second lens to a value in a 20 to 40 range, and an Abbe number of the third lens to a value in a 40 to 72 range, the chromatic aberration on the axis can be corrected well (see Patent Document 10).
However imaging lens belonging to the third category commonly comprises a third lens having a positive refractive power. Therefore in the case of imaging lenses belonging to the third category, increasing the ratio of the back focus to the optical length is difficult.
Also imaging lenses belonging to the third category commonly comprises a first lens having a positive refractive power, so the second lens is set to have a negative refractive power in order to correct aberrations.
Also the Abbe number of the second lens is set to a value less than 50. This makes the range of the selection of parameters (e.g. radius of curvature of lens surface) for defining the shape of the second lens become narrow. If the range of the selection of parameters for defining the shape of the second lens becomes narrow, compactness is limited accordingly.
As three lens-configuration imaging lenses belonging to the fourth category, the following eleventh to fifteenth imaging lenses have been disclosed (see Patent Documents 11 to 15).
The eleventh imaging lens is comprised of a first lens, an aperture stop, a second lens and a third lens arranged in this sequence from the object side. The first lens is a meniscus lens having a positive refractive power, of which convex surface is facing the object side. The second lens is a meniscus lens having a positive refractive power, of which concave surface is facing the image side and of which shape is such that both surfaces are aspherical, the negative refractive force gradually decreases as the edge section has a positive refractive power. Because of this configuration, an imaging lens, of which overall length is short with a small number of lenses, which has good optical performance that can be applied to the latest image sensors even though the lens system is compact, and which can decrease the incident angle to the image sensor, is implemented (see Patent Document 11).
The twelfth imaging lens is comprised of an aperture stop, a first lens which is a biconvex lens having a positive refractive power, a second lens having a negative refractive power, of which concave surface is facing the object side, and a third lens, which is a meniscus lens of which convex surface is facing the object side, arranged in this sequence from the object side. Because of this configuration, an imaging lens, which is smaller than a conventional type but in which various aberrations are corrected well, can be implemented (see Patent Documented 12).
The thirteenth imaging lens is comprised of a first lens, aperture stop, a second lens and a third lens. The first lens is a lens having a positive refractive power, of which at least one surface is aspherical, and a having a shape near the optical axis that is biconvex. The second lens is meniscus lens having a positive refractive power, of which at least one surface is aspherical, and concave surface is facing the object side near the optical axis. The third lens is a lens having a positive or negative refractive power formed of plastic material, of which both surfaces are aspherical and surface at the object side is a convex shape near the optical axis. Because of this configuration, a high performance and compact imaging lens is implemented by using aspherical surfaces effectively, with a smaller number of lenses while decreasing cost (see Patent Document 13).
The fourteenth imaging lens is comprised of the first lens, a aperture stop, a second lens, and a third lens arranged from the object side. The first lens is a lens having a positive refractive power, of which convex surface is facing the object side. The second lens is a meniscus lens having a positive refractive power, of which convex side is facing the image side. The third lens is a lens having a negative refractive power, of which concave surface is facing the image side. Because of this configuration, an image lens, which is smaller than a conventional type but in which various aberrations are corrected well, can be implemented (see Patent Documented 14).
The fifteenth imaging lens is comprised of an aperture stop, a first lens having a positive refractive power, a second lens having a negative refractive power, and a third lens having a positive or negative refractive power, arranged in this sequence from the object side. Because of this configuration, an imaging lens, which has good image formation performance, distortion aberration characteristic, sufficient peripheral light quantity and appropriate back focus, and which is also compact, can be implemented (see Patent Document 15).
However in the case of the eleventh to fifteenth imaging lenses belonging to the fourth category, the radius of curvature r1, near the optical axis on the object side face, of the first lens disposed at the object side out of the three composing lenses, is too large compared with the focal length (composite focal length determined by three lenses: first lens, second lens and third lens) f of the imaging lens, and it is difficult to design the imaging lens to be compact. In other words, the imaging lens belonging to the fourth category, the value r1/f is too large, therefore compacting the imaging lens is difficult. Compacting here quantitatively refers to decreasing the ratio of the focal length f of the imaging lens to the optical length D, that is D/f.
As a three lens-configuration imaging lens belonging to the fifth category, the following sixteenth imaging lens has been disclosed (see Patent Document 16).
The sixteenth imaging lens is comprised of a first lens, an aperture stop, a second lens and a third lens, arranged in this sequence from the object side. The first lens is a lens formed of a glass material, having a positive refractive power, of which the surface of the object side is a convex shape. The second lens is a meniscus lens having a positive refractive power formed of a plastic material, of which at least one surface is aspherical and concave surface is facing the object side. The third lens is a meniscus lens formed of a plastic material having a positive or negative refractive power, or which both surfaces are aspherical and convex surface is facing the object side. Because of this configuration, a high performance and compact imaging lens can be implemented while decreasing the cost (see Patent Document 16).
However the value of the focal length f1 of the first lens of the sixteenth imaging lens is too large with respect to the focal length f of the imaging lens (value f1/f is too large), so compacting this imaging lens is difficult.
As a three lens-configuration imaging lens belonging to the sixth category, the following seventeenth imaging lens has been disclosed (see Patent Document 17).
The seventeenth imaging lens is comprised of a first lens having a positive refractive power, of which concave surface is formed at the object side near the optical axis, an aperture stop, a second lens having a negative refractive power, and a third lens having a positive refractive power, and arranged in this sequence from the object side. The absolute value of the radius of the center curvature R1 of the first surface of the first lens at the object side is set to a focal length f of the entire optical system or more, and the focal length f2 of the second lens is set to 0.6 times the focal length f of the entire system or less. By specifying the radius of the center curvature R1 of the first surface of the first lens, each aberration, particularly the image surface curvature, can be effectively corrected. And even if the image sensor is small, the angle of view can be increased and the focal length can be decreased while maintaining a desired optical performance, and the entire optical system can be downsized and easily manufactured (see Patent Document 17).
However in the case of the seventeenth imaging lens, the first lens is a lens having a positive refractive power, of which concave surface is formed at the object side, and is set to be |R1|≧f1, so the back focus can be increased but decreasing the optical length D (decreasing the value D/f) is difficult. Therefore compacting the seventeenth imaging lens is difficult. Also the second lens has a negative refractive power and the Abbe number is set to be less than 50, so flexibility in design to decrease chromatic aberration is small, which makes it difficult to make the lens compact.
As a three lens-configuration imaging lens belonging to the seventh category, the following eighteenth imaging lens has been disclosed (see Patent Document 18).
The eighteenth imaging lens is comprised of a front group, where the first and second lens are disposed and which has a positive refractive power as an entire group, an aperture stop, and a rear group, where a third lens having a negative refractive power is disposed, arranged in this sequence from the object side. The first lens is a biconcave lens having a negative refractive power, of which concave surface is facing the object side. The second lens is a biconvex lens, of which surface having a strong curvature is facing the object side. The third lens is a meniscus lens having a negative refractive power, of which concave surface is facing the object side. Because of this configuration, an inexpensive and high performance compact lens, of which overall length of the lens (optical length) is short, can be implemented for the camera lens (see Patent Document 18).
However in the case of the eighteenth imaging lens, of which first lens has a negative refractive power, the value of a spherical aberration when the F number is 6.7 is about −1 mm, but the image surface curvature is large, about 1 mm, of which + or − is opposite that of the value of the spherical aberration. Because of these aberrations, the eighteenth imaging lens can be used for a conventional imaging using silver film, but cannot be mounted for a portable telephone which uses a CCD image sensor or a CMOS image sensor as an image sensor.
As a three lens-configuration imaging lens belonging to the eighth category, the following nineteenth imaging lens has been disclosed (see Patent Document 19).
The nineteenth imaging lens is comprised of a first lens having a positive refractive power of which convex surface is facing the object side, an aperture stop, a second lens which is a meniscus lens having a positive refractive power, of which convex surface is facing the image side, and a third lens having a negative refractive power of which concave surface is facing the image side. By adjusting parameters, such as the radius of curvature of the composing lens surface and space between composing lenses, using this configuration, a three lens-configuration imaging lens, which is smaller than a conventional type but where various aberrations are corrected well, can be implemented, and it becomes possible to provide imaging units and portable terminals where this imaging lens is mounted (see Patent Document 19).
However in the case of the nineteenth imaging lens, the ratio of the focal length f3 of the third lens to the focal length f of the imaging lens, that is (f3/f), is set to a value in a −2.0 to −0.4 range, and the negative refractive power of the third lens is strong (focal length is short). Therefore it is difficult to implement an imaging lens to provide good images where the open F value is smaller and various aberrations are sufficiently removed. According to the embodiment disclosed in Patent Document 19, the smallest open F value of the imaging lens is still 3.60.
As described above, in all the imaging lenses belonging to the first to eighth categories, lenses which satisfy the single condition that the optical length with respect to the focal length f of the imaging lens is short, or the back focus is at the maximum, are disclosed, but imaging lenses having sufficient brightness that simultaneously satisfies the following three conditions that the optical length is short, the back focus is at the maximum, and good images can be acquired, are not disclosed.
Even if the first to nineteenth imaging lenses are combined, it is difficult to implement an imaging lens having sufficient brightness, which can simultaneously satisfy the three conditions that the optical length is short, the back focus is at the maximum, and good images can be acquired.
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